This invention relates to plasma channels, to display devices comprising plasma channels, and to plasma-addressed liquid crystal display panels commonly referred to as xe2x80x9cPALCxe2x80x9d display devices using such channels. PALC devices comprise, typically, a sandwich of: a first substrate having deposited on it parallel transparent column electrodes, commonly referred to as xe2x80x9cITOxe2x80x9d columns or electrodes since indium-tin oxides are typically used, on which is deposited a color filter layer; a second substrate comprising parallel sealed plasma channels corresponding to -rows of the display crossing all of the ITO columns and each of which is filled with a low pressure ionizable gas, such as helium, neon and/or argon, and containing spaced cathode and anode electrodes along the channel for ionizing the gas to create a plasma, which channels are closed off by a thin -transparent dielectric sheet; and a liquid crystal (LC) material located between the substrates. The structure behaves like an active matrix liquid crystal display in which the thin film transistor switches at each pixel are replaced by a plasma channel acting as a row switch and capable of selectively addressing a row of LC pixel elements. In operation, successive lines of data signals representing an image to be displayed are sampled at column positions and the sampled data voltages are respectively applied to the ITO columns. All but one of the row plasma channels are in the de-ionized or non-conducting state. The plasma of the one ionized selected channel is conducting and, in effect, establishes a reference potential on the adjacent side of a row of pixels of the LC layer, causing each LC pixel to charge up to the applied column potential of the data signal. The ionized channel is turned off, isolating the LC pixel charge and storing the data voltage for a frame period. When the next row of data appears on the ITO columns, only the succeeding plasma channel row is ionized to store the data voltages in the succeeding row of LC pixels, and so on. As is well known, the attenuation of the backlight or incident light to each LC pixel is a function of the stored voltage across the pixel. A more detailed description is unnecessary because the construction, fabrication, and operation of such PALC devices have been described in detail in the following U.S. patents and publication, the contents of which are hereby incorporated by reference: U.S. Pat. Nos. 4,896,149; 5,077,553; 5,272,472; 5,276,384; and Buzak et al., xe2x80x9cA 16-Inch Full Color Plasma Addressed Liquid Crystal Displayxe2x80x9d, Digest of Tech. Papers, 1993 SID Int. Symp., Soc. for Info. Displ. pp. 883-886.
A partial perspective view of the PALC display described in the 1993 SID Digest is shown in FIG. 2. The method described in the referenced publication for making the plasma channels is to chemically etch a flat glass substrate to form parallel semi-cylindrically shaped recesses defined by spaced ridges or mesas and to bond on top of the mesas a thin dielectric cover sheet having a thickness in the range of about 30-50xcexcm.
The above construction and its fabrication encounters certain problems. Since the channel electrodes must be patterned on the sloping sidewall of the channel, the dimensions and placement of the electrodes cannot be accurately controlled. Moreover, since slight variations in processing conditions can alter the etch rate, the channel etching process is difficult to control; hence the depth of the channel, which is dependent on control of the etching process, is difficult to control.
European Patent 0 500 084 A2 describes the formation of channels by patterning of electrodes on a flat substrate, providing spacers on the flat substrate, and placing the thin glass sheet on top of the spacers. The discharge space thus extends continuously across the-electrodes. However, the continuous discharge space will lead between channels to crosstalk which is difficult to avoid. Moreover, the spacers have to be formed on the flat substrate by deposition and/or etching processes, such as screen printing. Since the spacers have to be as thick as the-required channel depth (xcx9c100 microns or more) the fabrication of the spacers adds complexity to the process.
European Patents 0 500 085 A2 and 0554 851 A1 describe the formation of channels by screen printing partition walls. However, this is also a difficult process, which may require multiple coats to obtain the required wall height.
An object of the invention is an improved channel plate.
A further object of the invention is an improved plasma-addressed display device.
Another object of the invention is an improved method for fabricating the plasma channels of a PALC display device.
In accordance with a first aspect of the invention, a channel plate comprises a dielectric substrate and a thin dielectric sheet-like member arranged over and spaced from the substrate by a plurality of laterally spaced, channel-defining spacer members each formed as part of a dielectric sheet patterned by through-holes, which latter sheet is herein referred to as the spacer sheet or plate. The holes are configured to form the desired channel configurations, typically elongated parallel channels, which preferably are straight but which also may be curved while still maintaining a substantially parallel relationship. The height of the spacer sheet above the substrate determines the height of the channels, which are each formed by the portion of the substrate surface extending between adjacent flanking spacers, the flanking spacers themselves forming the channel walls, and the overlying portion of the thin dielectric sheet-like member. Spaced electrodes are provided in each channel as well as a plasma-forming atmosphere. The channels are formed when the three sheet-like membersxe2x80x94the substrate, the spacer plate, and the thin dielectric sheetxe2x80x94are assembled and bonded together. By locating the spacer walls between the electrodes, so that the walls contact directly the substrate surface or an ion-forming layer on the substrate, the three sheet-like members can be attached by anodic bonding, a well-known process using heat and an electric field to cause mobile ions in the contacting materials to migrate to the sheet interfaces and bond them together.
In accordance with a second aspect of the invention, an etch stop layer is provided on the facing surface of the substrate or on the facing surface of the thin dielectric sheet-like member, the spacer plate attached to the member containing the etch stop layer, and the etching conducted in situ using an etch mask on the exposed surface of the spacer plate, the etchant penetration into the sheet containing the etch stop automatically stopping when the etch stop layer is reached. This simplifies not only the etching step but also simplifies handling of these fragile sheet-like members.
In accordance with a third aspect of the invention, the thin dielectric sheet-like member, as a separate element, can be avoided by depositing on the surface of the spacer plate a continuous layer or layers comprising an etch stop material to a thickness sufficient for the deposited layer to span without breaking or other damage the through-holes etched in the spacer plate and to seal off the plasma-forming atmosphere subsequently introduced into the channels. The deposited layer or layers thereby forms the required thin dielectric cover sheet for the channels.
In accordance with a first preferred embodiment of the invention, the substrate is of glass, the thin dielectric sheet is of glass, and the spacer sheet is a glass plate, with the through-holes formed by chemical or plasma etching or by mechanical means such as sandblasting. The three glass members may be bonded together using fused glass frit as described in several of the cited patents and publications, or by anodic bonding as described in the first related patent application identified above.
In accordance with another preferred embodiment of the invention, the channel plate is part of a PALC display device, and the combination of the substrate, patterned spacer plate and the overlying thin dielectric sheet-like member, together with the electrodes, constitutes the plasma channels or channel plate of the PALC display device.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its use, reference should be had to the accompanying drawings and descriptive matter in which there are illustrated and described the preferred embodiments of the invention, like reference numerals or letters signifying the same or similar components.